1. Field of the Invention
This invention relates broadly to self-propelled projectiles. More particularly, this invention relates to rockets powered by both solid propellant and hybrid propellant systems.
2. State of the Art
Rocket boosters (motors) generally fall into three classes: solid propellant boosters in which a solid fuel element, or grain, undergoes combustion to produce thrust that propels the rocket, liquid propellant boosters that accomplish the same function with a liquid fuel material, and hybrid boosters, described below. Solid and liquid rocket boosters can produce relatively large amounts of thrust, but for a relatively short amount of time. In addition, solid and liquid rocket boosters are generally expensive to develop and produce due to the inherent dangers of the highly combustible solid fuels.
Hybrid rocket boosters are described in detail in co-owned U.S. Pat. No. 5,715,675 to Smith et al., which is hereby incorporated by reference herein in its entirety. They have been characterized as a cross between a solid propellant booster and a liquid propellant booster. Generally hybrid boosters use a fluid reactant (oxidizer) to burn a solid fuel element, although they may use a combustible liquid fuel and a solid reactant. The hybrid rocket propellant (fuel and reactant together) can be ignited by an igniter, such as an electrically-generated spark, by pyrotechnic means, or by initial injection of an ignition fluid which exothermically reacts with the fuel or reactant. The fuel of a hybrid rocket is inert until mixed with the reactant (oxidizer) in the presence of an igniter. As such, there is no danger of inadvertent and uncontrollable combustion. In addition, hybrid boosters have the advantage of easily being able to start, stop, and restart based on the release of the oxidizer. Moreover, regulation of the release of the oxidizer permits a prolonged period of thrust relative to solid propellant boosters. While hybrid rocket boosters generally do not have the high thrust of solid propellant rocket boosters, hybrid rockets are generally favored in situations where it is desired to tradeoff high initial thrust for long burn time, safety and control.
In addition, there is also a need for flight vehicles, such as rockets, for educational and military use. In particular, rockets for educational use would provide hands on experience for students in the development of payloads and permit result comparison with existing data. However, such rockets would need to satisfy several criteria. First, the cost per rocket would necessarily have to be relatively low as funds available for educational purposes are extremely limited. Second, the rocket would be required to carry a relatively high payload weight, e.g., a hundred pounds minimum. Third, the rocket would preferably reach a relatively high altitude; such as to the edge of space, which is approximately fifty miles. With respect to military tactical use, it will be appreciated that a rocket satisfying these requirements, e.g., low cost, sufficient payload weight, and high altitude, could also have use as targets and reentry vehicles for anti-ballistic and airborne laser programs; i.e., rockets could be SCUD simulators.
One potentially viable rocket for educational uses and disposable military uses is a surplus rocket utilizing standard solid-propellant booster motors. Such motors are inexpensive, widely available, inventory surplus items. Referring to prior art FIGS. 1 through 3, the rocket 10 generally has a solid fuel booster 12, a nozzle 14 aft of the booster 12, and a nose cone 16 payload carrying portion bolted to the forward end of the booster 12. The booster 12 includes a steel casing 20 housing a solid fuel grain 22 (in a wagon wheel configuration) and has a forward igniter port 24. The booster 12 includes a head end ignition basket 28 coupled to the igniter port 24. The ignition basket 28 has an igniter 30 (FIG. 3) for igniting the solid fuel 22. Between the casing 20 and the fuel grain 22 are provided insulation, inhibitor, liner, immobilizer, and resonance rods, collectively 26, primarily made from cellulose acetate and rubber, which are mostly required to bond the solid propellant grain 22 to the casing 20 and which are not consumed during the burning of the solid propellant. Fin panels 32 are provided proximal and peripheral the nozzle 14 for stabilizing the rocket 10 in flight.
Referring to prior art FIG. 4, the typical solid booster was developed as a short burn, high thrust (e.g., 3.0 seconds at 45000 lbf) heavy-lift motor. Such a booster can lift a payload weight of 200 pounds, and therefore has the ability to easily satisfy the minimum payload requirements for educational and military use. However, the booster, which burns out when the rocket is at 7000 feet at a velocity of 4000 feet per second (mach 3.6), falls short on the altitude requirement, reaching only an altitude of about twenty-one miles. Therefore, while a rocket using the solid booster satisfies the requirement of a heavy-lift flight vehicle, it is nevertheless insufficient for either educational and particular military purposes.